The invention relates to a method for controlling an electrical machine and to an inverter, the electrical machine being connected to the inverter comprising phase-specific switching components generating an output voltage, an optimum switching table arranged to select a switching combination for the switching components on the basis of the stator flux and torque of the electrical machine, the method comprising steps in which the stator current vector and the rotation speed of the electrical machine are determined.
To reliably control electrical machines at a wide speed range without a direct feedback on the rotation speed or position angle causes considerable problems in the present control systems of machines. Especially when the electrical machine is a synchronous motor, the control of the rotation speed in all speed ranges is difficult due to the inaccuracy of the estimation of the stator flux which is the base for many control systems. This problem is especially enhanced when operating at low rotation speeds. A rotating-field machine, such as a synchronous machine, can be started using modern control methods based on direct torque control, but the load properties of the machine remain inadequate at low rotation speeds. In practice, it is, however, not possible to use methods based on direct torque control continuously at zero speed and low rotation speeds without feedback on the rotor position angle.
An error forms between the actual stator flux and the estimate formed of it in drives based on direct torque control. In such drives, the drifting of the stator flux is usually corrected by using a current model made for the machine. The use of a current model cannot, however, prevent a static torque error. A rotating-field machine can be controlled reliably at low rotation speeds by using stator current control in which the operation of the machine is controlled on the basis of the stator current.
It is an object of the present invention to produce a method avoiding the above-mentioned drawbacks and making it possible to reliably control an electrical machine at a wide rotation speed range utilizing a transfer from one control method to another when the rotation speed changes. This object is achieved by a method of the invention characterized in that the method also comprises a step in which a switching combination is defined for switching components on the basis of a stator current vector and a stator current reference vector of the electrical machine when the determined rotation speed is lower than a predefined limit.
The method of the invention is based on the idea that at higher rotation speeds the electrical machine is reliably controlled utilizing a control method based on direct torque control, but at lower rotation speeds the control method is changed to one based on stator current control. The change of control method provides a considerable advantage in that the drive remains stable even at low speeds.
The invention also relates to an inverter comprising phase-specific switching components generating an output voltage, an optimum switching table arranged to select a switching combination for the switching components on the basis of defining values, and a frequency definition element arranged to determine the rotation speed of the electrical machine controlled by the inverter. The inverter of the invention is characterized in that the inverter also comprises a selection element arranged to change the defining values used as the basis for the switching combination for the switching components depending on the rotation speed of the electrical machine.
The inverter of the invention is based on the idea that when the rotation speed of the electrical machine exceeds a given limit value, the inverter changes the defining variables used as a basis for modulation.
By means of the inverter of the invention, the advantages provided by the method of the invention can be realized with a simple structure using the same modulator to generate output voltages both in a control method based on direct torque control and in one based on current control.